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diff --git a/library/stdarch/crates/core_arch/src/x86_64/abm.rs b/library/stdarch/crates/core_arch/src/x86_64/abm.rs
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+//! Advanced Bit Manipulation (ABM) instructions
+//!
+//! The POPCNT and LZCNT have their own CPUID bits to indicate support.
+//!
+//! The references are:
+//!
+//! - [Intel 64 and IA-32 Architectures Software Developer's Manual Volume 2:
+//! Instruction Set Reference, A-Z][intel64_ref].
+//! - [AMD64 Architecture Programmer's Manual, Volume 3: General-Purpose and
+//! System Instructions][amd64_ref].
+//!
+//! [Wikipedia][wikipedia_bmi] provides a quick overview of the instructions
+//! available.
+//!
+//! [intel64_ref]: http://www.intel.de/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
+//! [amd64_ref]: http://support.amd.com/TechDocs/24594.pdf
+//! [wikipedia_bmi]:
+//! https://en.wikipedia.org/wiki/Bit_Manipulation_Instruction_Sets#ABM_.28Advanced_Bit_Manipulation.29
+
+#[cfg(test)]
+use stdarch_test::assert_instr;
+
+/// Counts the leading most significant zero bits.
+///
+/// When the operand is zero, it returns its size in bits.
+///
+/// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_lzcnt_u64)
+#[inline]
+#[target_feature(enable = "lzcnt")]
+#[cfg_attr(test, assert_instr(lzcnt))]
+#[stable(feature = "simd_x86", since = "1.27.0")]
+pub unsafe fn _lzcnt_u64(x: u64) -> u64 {
+    x.leading_zeros() as u64
+}
+
+/// Counts the bits that are set.
+///
+/// [Intel's documentation](https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_popcnt64)
+#[inline]
+#[target_feature(enable = "popcnt")]
+#[cfg_attr(test, assert_instr(popcnt))]
+#[stable(feature = "simd_x86", since = "1.27.0")]
+pub unsafe fn _popcnt64(x: i64) -> i32 {
+    x.count_ones() as i32
+}
+
+#[cfg(test)]
+mod tests {
+    use stdarch_test::simd_test;
+
+    use crate::core_arch::arch::x86_64::*;
+
+    #[simd_test(enable = "lzcnt")]
+    unsafe fn test_lzcnt_u64() {
+        assert_eq!(_lzcnt_u64(0b0101_1010), 57);
+    }
+
+    #[simd_test(enable = "popcnt")]
+    unsafe fn test_popcnt64() {
+        assert_eq!(_popcnt64(0b0101_1010), 4);
+    }
+}